Flex 6700 CW Keying v. Elecraft K3

Steve-N5AC

Let me explain what it going on here -- it's real subtle. There is latency in any receiver, most of it from filtering in the multiple stages of downconversion and demodulation (whether digital or analog). Let's say that the receiver is delayed in time by 40ms. If you are doing QSK keying and your dots are 60ms long, it will take 40ms after your dot for the RX to recover and then you will only hear 20ms of actual receiver noise in between the dits. As speeds increase and filters on CW get tighter, this problem gets worse making QRQ QSK impossible if you don't mix audio from the receiver and the side tone. We thought a lot about this and decided that the ear and brain are perfectly capable of filtering this out and by doing this we can provide QRQ QSK up to 100WPM. So this is how we implemented it. If you use a separate TX and RX antenna, the FLEX-6000 does a fine job of handing you RX audio and TX at the same time. To prevent any overload in the RX during the precise moment that a tone goes out, we mute the receiver. But this muting occurs in the digital domain and is timed precisely so that there is no TX audio present in the receiver. Since there is no AGC in the RF section of a FLEX receiver, there is no recovery period for the receiver to "come back down to earth" after hearing its own transmit signal.

Steve-N5AC

Exactly. This is why the CW engine in the FLEX-6000 is built for full remote with separate keying, sidetone synthesis and RF keying components.

Steve-N5AC

Incidentally we broke QSK CW in the Beta v1.0.x. Performance in v1.1 will be the same as the production v1.0 software.

Steve-N5AC

It is done on purpose to make QRQ QSK possible -- see my note above for the details

Paul Christensen, W9AC

Steve, Great explanation and it makes perfect sense. Paul, W9AC

Barry N1EU

Wow, as an occasionally serious cw contester, I take my hat off to what you guys have accomplished!

Steve Ellington

I'm 5 months late on this. The band noise at the onset of the sidetone can be annoying but I've done a few things to help reduce the effect. 1. Installed a separate ethernet card just for the 6300. This seems to have reduced latency vs. the router connection. 2. Turn the master volume up high and control volume level with the slice volume control. I'm not sure why this help. 3. Use an external audio filter for headphones seems to help but of course filters how their downsides. 4. Slide the L - R balance control partially to one side or entirely. Sidetone from one speaker will be pure and the other speaker will contain both. My ears found the switching effect less distracting. 5. Keep the bandwidth as narrow as practical. Obviously this reduces atmospheric noise. 6. Use APF and or peak the EQ to the sidetone frequency. Keep in mind what N5AC said about high speed QSK. If the band noise were to be muted during the beginning of the sidetone, QSK speed would be effectively reduced and you could miss a transmission. I do think that once a certain amount of "Delay" is inserted by the user we could do without the sidetone noise. This would make operating a bit more pleasant for those not interested in QSK. 73 Steve N4LQ